Electroluminescent Element and Light-Emitting Device

ABSTRACT

An electroluminescent element which can easily control the balance of color in white emission (white balance) is provided according to the present invention. The electroluminescent element comprises a first light-emitting layer containing one kind or two or more kinds of light-emitting materials, and a second light-emitting layer containing two kinds of light-emitting materials (a host material and a phosphorescent material) in which the phosphorescent material is doped at a concentration of from 10 to 40 wt %, preferably, from 12.5 to 20 wt %. Consequently, blue emission can be obtained from the first light-emitting layer and green and red (or orange) emission can be obtained from the second light-emitting layer. An electroluminescent element having such device configuration can easily control white balance since emission peak intensity changes at the same rate in case of increasing a current density.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/983,354, filed Jan. 3, 2011, now allowed, which is a continuation ofU.S. application Ser. No. 10/819,282, filed Apr. 7, 2004, now U.S. Pat.No. 7,862,906, which claims the benefit of a foreign priorityapplication filed in Japan as Serial No. 2003-105135 on Apr. 9, 2003,all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electroluminescent elementcomprising an anode, a cathode, and a layer including organic compounds(hereinafter, an electroluminescent layer) which emit light by applyingcurrent through the pair of electrodes; and a light-emitting devicewhich comprises the electroluminescent layer. More specifically, thisinvention relates to an electroluminescent element which exhibits whitelight emission, and a full color light-emitting device comprising theelectroluminescent element.

2. Related Art

An electroluminescent element comprises an electroluminescent layerinterposed between a pair of electrodes (anode and cathode). Theemission mechanism is as follows. Upon applying voltage through the pairof electrodes, holes injected from an anode and electrons injected froma cathode are recombined with each other within the electroluminescentlayer to result in the formation of molecular excitons, and themolecular excitons return to the ground state while radiating energy toemit photon. There are two excited states possible from organiccompounds, a singlet state and a triplet state. It is considered thatlight emission is possible through both the singlet state and thetriplet state.

Although an electroluminescent layer may have a single layer structurecomprising only a light-emitting layer formed by a light-emittingmaterial, the electroluminescent layer is formed to have not only asingle layer structure comprising only a light-emitting layer but also alamination layer structure having a hole injecting layer, a holetransporting layer, a hole blocking layer, an electron transportinglayer, an electron injecting layer, and the like which are formed by aplurality of functional materials.

It is known that color tone can be appropriately changed by doping tinyamounts of fluorescent substances (typically, at most approximately 10⁻³mol % based on the value of host substances) into host substances withinthe light-emitting layer. (For example, refer to Japanese PatentPublication No. 2,814,435.)

Besides, the following are known as methods for changing color tone.Blue light emission obtained from a light-emitting layer is used as alight emission source, and the obtained emission color is converted intodesired color within a color changing layer formed by color changingmaterials (hereinafter, CCM method). Alternatively, white light emissionobtained from a light-emitting layer is used as a light emission source,and the obtained emission color is converted into desired color by acolor filter (hereinafter, CF method).

However, in case of adopting the CCM method, there has been a problem inred color since color conversion efficiency of from blue to red is poorin principle. In addition, there has been a problem that the contrastbecomes deteriorated from light emission in pixels due to outside lightsuch as sunlight since color conversion materials are fluorescentmaterials. Therefore, it is considered that CF method without suchproblems is preferably used.

In the case of using CF method, an electroluminescent element exhibitingwhite light emission (hereinafter, white light emission element) havinghigh luminance is required since much light is absorbed in a colorfilter.

With respect to the white light emission element, elements formed byvarious materials to have various configurations have been reported. Itis quite important to control the balance of emission color (whitebalance) since white light emission is obtained by a plurality ofmaterials, each of which emits different color, meanwhile it isdifficult to do that.

For example, in the case that white light emission is obtained by mixinga plurality of materials, each of which exhibits emission in blue, greenand red colors, it has been reported that an emission peak intensity ofeach the materials is different depending on current density. (Forexample, refer to Brien W. D'Andrede, Jason Brooks, Vadim Adamovich,Mark E. Thompson, and Stephen R. Forrest, Advanced Material (2002), 14,No. 15, Aug. 5, 1032-1036 (FIG. 2)) In case of forming such element,peak intensity of each emission color changes in different rates.Consequently, it becomes extremely difficult to control white balancewhich is adjusted by the parameter which determines the peak intensityof these emission colors.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide anelectroluminescent element which can be easily controlled the colorbalance in white light emission (white balance).

As the result of research for solving the above mentioned problems, theinventor found that the rate of changes of peak intensity becomes hardlychanged depending on light-emitting materials even when a currentdensity is increased by keeping the concentration of light-emittingmaterials among a plurality of light-emitting materials used forobtaining white light emission in a certain range.

In view of the foregoing, an electroluminescent layer in anelectroluminescent element comprises a first light-emitting layercontaining one kind or two or more kinds of light-emitting materials;and a second light-emitting layer containing two kinds of light-emittingmaterials in which one of them is doped at a concentration of from 10 to40 wt %, preferably, from 12.5 to 20 wt %.

In the above described structure, blue emission having an emission peakintensity in the wavelength region of from 400 to 500 nm can be obtainedfrom the first light-emitting layer by using one kind or two or morekinds of light-emitting materials; and green emission having an emissionpeak intensity in the wavelength region of from 500 to 550 tun and red(or orange) emission having an emission peak intensity in the wavelengthregion of from 550 to 700 nm can be obtained from the secondlight-emitting layer by using two kinds of light-emitting materials,host materials and phosphorescent materials, to dope phosphorescentmaterials at a concentration of from 10 to 40 wt %, preferably, 12.5 to20 wt %. However, as the phosphorescent materials, materials which canresult in the formation of an excited dimer (excimer) formed by the bondof excited atoms or molecules with ground state atoms or molecules areused here.

As stated above, the first light-emitting layer exhibiting blue emissionand the second light-emitting layer exhibiting green and red (or orange)emission are formed separately. In consequence, there is an advantagethat the electroluminescent element becomes easy to manufacture sincethe control of the concentration or the like for forming the firstlight-emitting layer becomes facilitated. Further, compared with thecase that all color emission is obtained from one light-emitting layer,it can be expected that the varies of emission wavelength, the decreaseof peak intensity, or the like due to the interaction of moleculeshaving different structures from each other (exciplex) within alight-emitting layer is prevented.

Further, by doping phosphorescent materials into the secondlight-emitting layer within the above described concentration ranges,not only the number of excimer formed from phosphorescent materials canbe controlled, but also the light emission (green emission and redemission) can be obtained from the second light-emitting layersimultaneously with blue emission from the first light-emitting layer.In this case, the peak intensity of phosphorescent emission (greenemission) obtained from phosphorescent materials and emission (red (ororange)) from excimer (the peak intensity of both the emission can beconsidered to be the same because the intensity ratio is depending onthe concentration) and the peak intensity of blue emission from thefirst light-emitting layer changes at the almost same rate in case ofincreasing current density, hence, the peak intensity is easy to controland white light emission with well white balanced can be easilyobtained.

Therefore, one of constituent features of the invention is that anelectroluminescent element comprises an electroluminescent layerinterposed between a pair of electrodes wherein the electroluminescentlayer comprises at least a first light-emitting layer and a secondlight-emitting layer, each of which has an emission peak in a wavelengthregion of from 500 to 700 nm; and the second light-emitting layercontains a phosphorescent material which forms excimer at aconcentration of from 10 to 40 wt %, preferably, from 12.5 to 20 wt %.

Another constituent features of the invention is that anelectroluminescent element comprises an electroluminescent layerinterposed between a pair of electrodes wherein the electroluminescentlayer comprises at least a first light-emitting layer and a secondlight-emitting layer, each of which has an emission peak in a wavelengthregion of from 500 to 700 nm; and the second light-emitting layercontains a phosphorescent material which forms excimer at aconcentration of at least 10⁻⁴ mol/cm³ and at most 10⁻³ mol/cm³.

By setting a concentration of the phosphorescent material within theabove described ranges, an emission peak intensity of green lightemission (occurred in a wavelength region of from 500 to 550 nm) to red(or orange) light emission (occurred in a wavelength region of from 550to 700 nm) has a ratio of from 50 to 150%, preferably, from 70 to 130%.

Additionally, by setting a concentration of the phosphorescent materialwithin the above described ranges, a luminance of from 100 to 2000cd/m², preferably, from 300 to 1000 cd/m² is obtained from theelectroluminescent element.

More additionally, by setting a concentration of the phosphorescentmaterial within the above described ranges, a part of the phosphorescentmaterial can be existed at a certain distance from one another so thatthe molecules of the phosphorescent material can form excimer emission.

Besides, by setting a concentration of the phosphorescent materialwithin the above described ranges, the phosphorescent material formed bya metal complex can be existed so that central metals of thephosphorescent materials are a distance of from 2 to 20 Å from oneanother.

In the above each constituent features, the second light-emitting layeris formed to have preferably the thickness of from 20 to 50 nm, morepreferably, from 25 to 40 nm.

Further, in the above each constituent features, the firstlight-emitting layer has an emission spectrum with an emission peak in awavelength region of from 400 to 500 nm; the second light-emitting layerhas an emission spectrum with an emission peak in a wavelength region offrom 500 to 700 nm; and any one of the plurality of emission peaks isexcimer emission.

In the above each constituent features, the phosphorescent material isorganic metal complex with platinum as a central metal.

The invention comprehends an electric appliance and a light-emittingdevice, each of which comprises the above described electroluminescentelement.

Accordingly, an electroluminescent element can be provided which can becontrolled easily the color balance in white light emission (whitebalance) by keeping the concentration so as to be within a certain rangeof light-emitting materials among a plurality of light-emittingmaterials used for obtaining white light emission.

These and other objects, features and advantages of the invention willbecome more apparent upon reading of the following detailed descriptionalong with the accompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are views showing light emission mechanism of anelectroluminescent element according to the present invention;

FIGS. 2A and 2B are band diagrams showing a device configuration of anelectroluminescent element according to the invention;

FIGS. 3A and 3B are band diagrams showing a device configuration of anelectroluminescent element according to the invention;

FIGS. 4A and 4B are band diagrams showing a device configuration of anelectroluminescent element according to the invention;

FIG. 5 is a view showing a specific device configuration of anelectroluminescent element according to the invention;

FIGS. 6A and 6B are schematic views of a light-emitting device accordingto the invention;

FIGS. 7A to 7G are examples of electric appliances using alight-emitting device according to the invention;

FIG. 8 shows an emission spectrum according to Example 2 and ComparativeExample 1;

FIG. 9 shows current density dependence of an emission spectrumaccording to Example 2;

FIG. 10 shows luminance-current characteristics according to Example 2and Comparative Example 1;

FIG. 11 shows luminance-voltage characteristics according to Example 2and Comparative Example 1;

FIG. 12 shows current efficiency-current characteristics according toExample 2 and Comparative Example 1; and

FIG. 13 shows current-voltage characteristics according to Example 2 andComparative Example 1.

DESCRIPTION OF THE INVENTION

An electroluminescent element according to the present inventioncomprises a pair of electrodes (an anode and a cathode) and anelectroluminescent layer having at least a first light-emitting and asecond light-emitting layer interposed between the pair of electrodes.As shown in FIG. 1A, within the first light-emitting layer,light-emitting materials are excited by recombination of carriers, andmonomers in excited states are formed, then, light-emission is obtained(blue emission: hν₁). Within the second light-emitting layer,phosphorescent materials are excited by recombination of carriers, andmonomers in excited states are formed, then, phosphorescent emission isobtained (green emission: hν₂). Simultaneously, within the secondlight-emitting layer, monomer in the excited state and monomer in theground state form an excited dimer (excimer), and excimer emission (red(or orange): hν₂′) can also be obtained.

Within the second light-emitting layer, since an energy state of theexcimer state obtained from phosphorescent materials is lower than thatof the excited state obtained from phosphorescent materials as shown inFIG. 1B, the excimer emission (hν₂′) is always at longer wavelength sidethan the general phosphorescent light emission (hν₂) (specifically, atleast several ten nm longer wavelength side). Therefore, in the casethat phosphorescent materials which can generate phosphorescent emissionat a green emission wavelength region are used as in the invention,excimer emission is at the red emission wavelength region. Hence,according to the invention, by combining green emission and red emissionfrom phosphorescent materials with blue emission from anotherlight-emitting materials, high efficient white light emission havingpeak intensity in each red, green, and blue wavelength region can beobtained.

For forming an excimer state from phosphorescent materials, it isrequired to make it easier for monomer in an excited state and monomerin a ground state to form an excited dimer by their interaction.Specifically, phosphorescent materials are preferably doped into hostmaterials at concentration of from 10 wt % to 40 wt %, more preferably,from 12.5 wt % to 20 wt % within the second light-emitting layer.Besides, phosphorescent materials having high planarity structures suchas platinum complex are preferably used as guest materials to keep thedistance between central ions (or atoms) of the phosphorescent materialswithin a certain range. According to the invention, in the case that thephosphorescent materials (monomer) in a ground state and phosphorescentmaterials (monomer) in an excimer state are respectively at the positiondenoted by (a) and the position denoted by (b) as shown in FIG. 1C, thedistance between central ions (d₁) is preferably from 2 to 5 Å. However,phosphorescent materials (monomer) in a ground state can interact withphosphorescent materials (monomer) in an excited state even when thephosphorescent materials (monomer) in a ground state are at the positiondenoted by (c). Further, in consideration that the average radius (r) ina molecular structure of phosphorescent materials according to theinvention is approximately from 6 to 9 Å, the distance between centralions (d₂) is preferably 2 Å≦d₂≦20 Å.

The first light-emitting layer exhibiting blue emission can be formed bya single substance (blue luminous body), or host materials and guestmaterials (blue luminous body).

For forming an electroluminescent element according to the invention, adevice design is required so that both the first light-emitting layerand the second light-emitting layer to emit light. Specifically, therelationship of ionization potential among the first light-emittinglayer, the second light-emitting layer, and another layers, each ofwhich composes an electroluminescent layer is required to be mostappropriate.

In addition, the device design becomes different depending on theconfiguration of functional layers composing an electroluminescentlayer, subsequently, preferred embodiment of the invention will bedescribed in terms of the relationship between a device configurationand a band diagram hereinafter.

Embodiment 1

In Embodiment 1, as shown in FIG. 2A, the case that a first electrode201, an electroluminescent layer 202, and a second electrode 203 areformed over a substrate 200, and that the electroluminescent layer 202has a lamination structure comprising a first light-emitting layer 211,a second light-emitting layer 212, and an electron transporting layer213 will be explained. In addition, the first light-emitting layer 211includes a light-emitting body. The second light-emitting layer 212comprises host materials 251 and phosphorescent materials 252 whichserve as a light-emitting body. The phosphorescent materials cangenerate both phosphorescent emission and excimer emission.

As a light-emitting body (light-emitting material) for the firstlight-emitting layer 211, blue fluorescent materials having holetransportation properties such asN,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine(abbreviated TPD) or derivatives thereof such as4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (hereafter, referred toas α-NPD); or blue fluorescent materials having electron transportationproperties such asbis(2-methyl-8-quinolinolate)-(4-hydroxy-biphenylyl)-aluminum(abbreviated BAlq) or bis[2-(2-hydroxyphenyl)-benzooxazolate]zinc(abbreviated Zn(BOX)₂). Various blue fluorescent dyes, for example,perylene, 9,10-diphenyl anthracene, or coumarin based fluorescent dyes(coumarin 30 or the like) can be used as guest materials. Further,phosphorescent materials such asbis(4,6-difluorophenyl)pyridinato-N,C²′) (acetylacetonato)iridium(abbreviated Ir(Fppy)₂(acac)) can be used. All of these materials haveemission peak intensity in the wavelength region of from 400 to 500 nm,so that they are suitable for materials for the light-emitting body ofthe first light-emitting layer 211 according to the invention.

An organic metal complex with platinum as a central metal is efficientlyused for a light-emitting body (phosphorescent material) of the secondlight-emitting layer 212. Specifically, by doping substances representedby the structural formulas 1 to 4 at concentration of from 10 wt % to 40wt %, preferably, from 12.5 wt % to 20 wt % into host materials, bothphosphorescent emission and its excimer emission can be obtained.However, the present invention is not limited thereto, anyphosphorescent material can be used as long as both phosphorescentemission and excimer emission can be simultaneously obtained therefrom.

In case of using guest materials for the first light-emitting layer andthe second light-emitting layer comprising a light-emitting body, holetransportation materials or electron transportation materials typifiedby the following examples can be used. In addition, bipolar materialssuch as 4,4′-N,N′-dicarbazolyl-biphenyl (abbreviated CBP) can be used ashost materials.

As hole transportation materials, aromatic amine (that is, the onehaving a benzene ring-nitrogen bond) compounds are preferably used. Forexample,N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine(abbreviated TPD) or derivatives thereof such as4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviated α-NPD) iswidely used. Also used are star burst aromatic amine compounds,including: 4,4′,4″-tris(N,N-diphenyl-amino)-triphenyl amine (abbreviatedTDATA); and 4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine (abbreviated MTDATA).

As electron transportation materials, metal complexes such astris(8-quinolinolate) aluminum (abbreviated Alq₃),tris(4-methyl-8-quinolinolate) aluminum (abbreviated Almq₃),bis(10-hydroxybenzo[h]-quinolinato) beryllium (abbreviated BeBq₂), BAlq,Zn(BOX)₂, and bis[2-(2-hydroxyphenyl)-benzothiazolate]zinc (abbreviatedZn(BTZ)₂). Additionally, oxadiazole derivatives such as2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviatedPBD), and 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviated OXD-7); triazole derivatives such as3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviated TAZ) and3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviated p-EtTAZ); imidazol derivatives such as2,2′,2″-(1,3,5-benzenetryil)tris[1-phenyl-1H-benzimidazole] (abbreviatedTPBI); and phenanthroline derivatives such as bathophenanthroline(abbreviated BPhen) and bathocuproin (abbreviated BCP) can be used inaddition to metal complexes.

In addition, the above described electron transporting materials can beused for the electron transporting layer 213.

FIG. 2B is a band diagram in case of forming a device having the abovedescribed configuration. The diagram shows a HOMO level (ionizationpotential) 220 of the first light-emitting layer 201; a HOMO level(ionization potential) 221 and a LUMO level 222 of the firstlight-emitting layer 211; a HOMO level (ionization potential) 223 and aLUMO level 224 of host materials of the second light-emitting layer 212in a thin film shape; a HOMO level (ionization potential) 225 and a LUMOlevel 226 of guest materials (phosphorescent materials) of the secondlight-emitting layer 212; a HOMO level (ionization potential) 227 and aLUMO level 228 of the electron transporting layer 213; and a LUMO level229 of the second electrode 203, respectively.

In this instance, an energy gap 230 between the ionization potential 221of the first light-emitting layer 211 and the ionization potential (inthis instance, the ionization potential of the host materials 251 isconsidered that of the whole second light-emitting layer 212) 223 of thewhole second light-emitting layer 212 (in the state that both the hostmaterials 251 and the phosphorescent materials 252 are included) ispreferably sufficiently large (specifically, at least 0.4 eV). If theenergy gap 230 is small, holes inject to the second light-emitting layer212 from the first light-emitting layer 211 since the firstlight-emitting layer 211 has hole transportation properties.Consequently, the majority of carriers are recombined within the secondlight-emitting layer 212. Therefore, since the second light-emittinglayer 212 exhibits emission of the wavelength region of green to red,energy cannot transfer to the first light-emitting layer 211 whichexhibits blue emission of further shorter wavelength. Accordingly, onlythe second light-emitting layer emits light.

By making the energy gap 230 sufficiently large, the majority ofcarriers are recombined with each other at the vicinity of an interfacebetween the first light-emitting layer 211 and the second light-emittinglayer 212. The other small number of carries are recombined within thesecond light-emitting layer 212, or trapped partly in the HOMO level 214of the phosphorescent materials, consequently, both the light-emittinglayer 211 and the second light-emitting layer 212 can emit light.

The same is true in the case that guest materials generating blueemission are contained in the host materials within the firstlight-emitting layer 211. That is, the energy gap 230 between theionization potential 221 of the whole light-emitting layer 211 (in thestate that both host materials of the first light-emitting layer and theguest materials generating blue emission are included) and theionization potential 223 of the whole second light-emitting layer 212(in the state that both the host materials 251 of the secondlight-emitting layer and the phosphorescent materials 252 are included)is preferably large (specifically, at least 0.4 eV).

Further, an energy gap 231 between the ionization potential 223 of thewhole second light-emitting layer 212 (in the state that both the hostmaterials 221 and the phosphorescent materials 222 are included) and theionization potential 227 of the electron transporting layer 213 ispreferably large (specifically, at least 0.4 eV). In this instance, bymaking the energy gap 231 large, holes serving as carriers can betrapped in the second light-emitting layer 212, hence, carriers can beefficiently recombined within the second light-emitting layer 212.

Moreover, an energy gap 232 between the LUMO level 222 of the firstlight-emitting layer 211 and the LUMO level (in this instance, the LUMOlevel of the host materials is considered that of the whole secondlight-emitting layer) 224 of the whole second light-emitting layer 212(in the state that both the host materials 251 and the phosphorescentmaterials 252 are included) is preferably large (specifically, at least0.4 eV). In this instance, by making the energy gap 232 large, holesserving as carriers can be trapped in the second light-emitting layer212, hence, recombination can be took place efficiently in the secondlight-emitting layer 212.

Therefore, light emission from the first light-emitting layer 211 andthe second light-emitting layer 212 can be further efficiently obtainedby a band-gap structure having energy gaps 230, 231, and 232 inEmbodiment 1.

Embodiment 2

In Embodiment 2, the case that a first electrode 301, anelectroluminescent layer 302, and a second electrode 303 are formed overa substrate 300, and the electroluminescent layer 302 has a laminationstructure comprising a first light-emitting layer 311 and a secondlight-emitting layer 312 will be explained with reference to FIG. 3A. Inaddition, second light-emitting layer 312 comprises host materials 351and phosphorescent materials 352. The phosphorescent materials cangenerate phosphorescent emission and excimer emission.

Embodiment 2 is distinguished from Embodiment 1 by the fact that theelectroluminescent layer does not comprise the electron transportinglayer, and so

Embodiment 2 has an advantage that the process for forming the electrontransporting layer can be omitted. In addition, materials havingexcellent electron transportation properties are preferably used as hostmaterials 321 for the second light-emitting layer 312 in order to keepluminous efficiency.

In Embodiment 2, materials for forming the first light-emitting layer311 and the second light-emitting layer 312 are the same as thosedescribed in Embodiment 1.

FIG. 3B is a band diagram in case of forming a device having the abovedescribed configuration. The diagram shows a HOMO level (ionizationpotential) 320 of the first light-emitting layer 301; a HOMO level(ionization potential) 321 and a LUMO level 322 of the firstlight-emitting layer 311; a HOMO level (ionization potential) 323 and aLUMO level 324 of host materials of the second light-emitting layer 312in a thin film shape; a HOMO level (ionization potential) 325 and a LUMOlevel 326 of guest materials (phosphorescent materials) of the secondlight-emitting layer 312; and a LUMO level 327 of the second electrode303, respectively.

In Embodiment 2 just as Embodiment 1, in order to obtain efficientlylight from the first light-emitting layer 311 and the secondlight-emitting layer 312, an energy gap 341 between the ionizationpotential 321 of the first light-emitting layer 311 and the ionizationpotential (in this instance, the ionization potential of the hostmaterials 351 is considered that of the whole second light-emittinglayer 312) 323 of the whole second light-emitting layer 312 (in thestate that both the host materials 351 and the phosphorescent materials352 are included) is preferably sufficiently large (specifically, atleast 0.4 eV). Further, an energy gap 342 between the LUMO level 322 ofthe first light-emitting layer 311 and the LUMO level (in this instance,the LUMO level of the host materials 351 is considered that of the wholesecond light-emitting layer 312) 324 of the whole second light-emittinglayer 312 (in the state that both the host materials 351 and thephosphorescent materials 352 are included) is preferably sufficientlylarge (specifically, at least 0.3 eV).

The same is true in the case that guest materials generating blueemission are contained in host materials within the first light-emittinglayer 311. That is, the energy gap 341 between the ionization potential(in this instance, the ionization potential of the host materials of thefirst light-emitting layer 311 is considered the ionization potential ofthe whole first light-emitting layer 311) 321 of the wholelight-emitting layer 311 (in the state that both the host materials andthe guest materials generating blue emission are included) and theionization potential 323 of the whole second light-emitting layer 312(in the state that both the host materials 351 and the phosphorescentmaterials 352 are included) is preferably large (specifically, at least0.4 eV).

Embodiment 3

In Embodiment 3, the case that a first electrode 401, anelectroluminescent layer 402, and a second electrode 403 are formed overa substrate 400, and that the electroluminescent layer 402 has alamination structure comprising a hole injecting layer 411, a firstlight-emitting layer 412, and a second light-emitting layer 413, and anelectron transporting layer 414 will be explained with reference to FIG.4A. In addition, second light-emitting layer 413 comprises hostmaterials 451 and phosphorescent materials 452 that serve as alight-emitting body. The phosphorescent materials can generatephosphorescent emission and excimer emission.

As materials for the hole injecting layer 411, besides the abovedescribed hole transporting materials such as TPD, α-NPD, TDATA, orMTDATA, the following hole transporting materials can be used.

As hole injection materials, porphyrin compounds are useful among otherorganic compounds such as phthalocyanine (abbreviated H₂-Pc), copperphthalocyanine (abbreviated Cu-Pc), or the like. Further, chemical-dopedconductive polymer compounds can be used, such as polyethylenedioxythipophene (abbreviated PEDOT) doped with polystyrene sulfonate(abbreviated PSS), polyaniline, or polyvinyl carbazole (abbreviatedPVK). A thin film of an inorganic semiconductor such as vanadiumpentoxide or an ultra thin film of an inorganic insulator such asaluminum oxide can also be used.

As a light-emitting body or materials for the first light-emitting layer412, a second light-emitting layer 413, and an electron transportinglayer 414, the same materials described in Embodiment 1 can be used,respectively.

FIG. 4B is a band diagram in case of forming a device having the abovedescribed configuration. The diagram shows a HOMO level (ionizationpotential) 420 of the first electrode 401; a HOMO level (ionizationpotential) 421 and a LUMO level 422 of the hole injecting layer 411; aHOMO level (ionization potential) 423 and a LUMO level 424 of the firstlight-emitting layer 412; a HOMO level (ionization potential) 425 and aLUMO level 426 of host materials of the second light-emitting layer 413;a HOMO level (ionization potential) 427 and a LUMO level 428 of guestmaterials (phosphorescent materials) of the second light-emitting layer413; a HOMO level (ionization potential) 429 and a LUMO level 430 of theelectron transporting layer 414; and a LUMO level 431 of the secondelectrode 403, respectively.

The configuration described in Embodiment 3 is distinguished from thatdescribed in Embodiment 1 by the fact that the hole injecting layer 411is included.

In Embodiment 3, in order to obtain light further efficiently from thefirst light-emitting layer 412 and the second light-emitting layer 413,an energy gap 441 between the ionization potential 423 of the firstlight-emitting layer 412 and the ionization potential (in this instance,the ionization potential of the host materials 451 is considered that ofthe whole second light-emitting layer 413) 425 of the whole secondlight-emitting layer 413 (in the state that both the host materials 451and the phosphorescent material 452 are included) is preferablysufficiently large (specifically, at least 0.4 eV). Likewise, an energygap 442 between the ionization potential 425 of the whole secondlight-emitting layer 413 (in the state that both the host materials 451and the phosphorescent material 452 are included) and the ionizationpotential 429 of the electron transporting layer 414 is preferablysufficiently large (specifically, at least 0.4 eV). Additionally, anenergy gap 443 between the LUMO level 424 of the first light-emittinglayer 412 and the LUMO level (in this instance, the LUMO level of thehost material 451 is considered that of the whole second light-emittinglayer 413) 426 of the whole second light-emitting layer 413 (in thestate that both the host materials 451 and the phosphorescent materials452) is preferably sufficiently large (specifically, at least 0.3 eV).Moreover, an energy gap 444 between the LUMO level 422 of the holeinjecting layer 411 and the LUMO level 423 of the first light-emittinglayer 412 is preferably sufficiently large (specifically, at least 0.3eV).

By holding the energy gap 444 between the LUMO level 422 of the holeinjecting layer 411 and the LUMO level 423 of the first light-emittinglayer 412, electrons can be trapped into the first light-emitting layer412, and so carriers can be efficiently recombined with each other inthe first light-emitting layer 412.

The same is true in the case that guest materials generating blueemission are contained in host materials within the first light-emittinglayer 412. That is, an energy gap 441 between the ionization potential(in this instance, the ionization potential of host materials of thefirst light-emitting layer is considered that of the whole firstlight-emitting layer 412) 423 of the whole first light-emitting layer412 (in the state that both the host materials and the guest materialsgenerating blue emission are included) and the ionization potential 425of the whole second light-emitting layer 413 (in the state that both thehost materials 451 and the phosphorescent materials 452 are included) ispreferably sufficiently large (specifically, at least 0.4 eV).

Accordingly, by applying the typical configurations described inEmbodiments 1 to 3 according to the present invention, a whiteelectroluminescent element having peak intensity in each wavelengthregion of red, green, and blue can be achieved with such a simple deviceconfiguration.

Further, the above described configuration is illustrative only as oneof preferred configurations. An electroluminescent layer in aelectroluminescent element according to the invention may comprise atleast the above described first light-emitting layer and secondlight-emitting layer. Therefore, though not nominated in thisspecification, a layer having properties except light-emissionproperties (for example, an electron injection layer, or the like) whichis known as used in the conventional electroluminescent element can beappropriately used.

As electron injection materials for forming an electron injecting layer,above described electron transportation materials can be used.Additionally, a ultra thin film of insulator, for example, alkalinemetal halogenated compounds such as LiF or CsF; alkaline earthhalogenated compounds such as CaF₂; or alkaline metal oxides such asLi₂O is often used. Further, alkaline metal complexes such as lithiumacetylacetonate (abbreviated Li(acac)), 8-quinolinolato-lithium(abbreviated Liq) can also be used.

An electroluminescent element according to the invention may be formedin such a way that at least either electrode is formed by transparentmaterials in order to extract light through either the electrode.Generally, the configuration that a first electrode formed over asubstrate is transparent to light (also referred to as a bottom emissionstructure); the configuration that a second electrode which is stackedover an electroluminescent layer formed over the first electrode istransparent to light (also referred to as a top emission structure); orthe configuration that both electrodes are transparent to light (alsoreferred to as a dual emission structure) may be adopted.

As anode materials for either the first electrodes (201, 301, and 401)or the second electrodes (203, 303, and 403), conductive materialshaving large work functions are preferably used. When light is extractedthrough an anode, transparent conductive materials such as indium-tinoxides (ITO) or indium-zinc oxides (IZO) may be used. When an anode isformed to have light shielding properties, a single layer formed by TIN,ZrN, Ti, W, Ni, Pt, Cr, or the like; a lamination layer comprising thesingle layer and a film containing titanium nitride and aluminum as itsmain components; a three lamination layer comprising a titanium nitridefilm, a film containing aluminum as its main components, and a titaniumnitride film; or the like can be used. Alternatively, the anode can beformed by stacking the above described transparent conductive materialsover a reflective electrode of Ti, Al, or the like.

As materials for a cathode, conductive materials having small workfunctions are preferably used. Specifically, alkaline metals such as Lior Cs; alkaline earth metals such as Mg, Ca, or Sr; alloys of thesesmetals (Mg:Ag, Al:Li, or the like); or rare earth metals such as Yb orEr can be used. In addition, in case of using an electron injectinglayer formed by LiF, CsF, CaF₂, Li₂O, or the like, the conventionalconductive thin film such as aluminum can be used. In case of extractinglight through cathode, the cathode can be formed to have a laminationstructure comprising a ultra thin film containing alkaline metals suchas Li or Cs, or alkaline earth metals such as Mg, Ca or Sr and atransparent conductive film (ITO, IZO, ZnO, or the like). Alternatively,the cathode may be formed by forming an electron injecting layer byalkaline metals or alkaline earth metals, forming electrontransportation materials by co-evaporation, and stacking a transparentconductive film (ITO, IZO, ZnO, or the like) thereon.

In manufacturing the above described electroluminescent elementaccording to the invention, a method for stacking each layer of theelectroluminescent element is not limited. Any method of vacuum vapordeposition, spin coating, ink jetting, dip coating, or the like can beused, as long as layers can be stacked by these methods.

Example 1

Hereinafter, examples of the present invention will be explained.

In this example, a device configuration of an electroluminescent elementand a method for manufacturing thereof according to the invention willbe explained with reference to FIG. 5.

An anode 501 of the electroluminescent element was formed over a glasssubstrate 500 having an insulating surface. As a material for the anode501, ITO, a transparent conductive film, is used. The anode 501 isformed by sputtering to have a thickness of 110 mm. The anode 501 issquare in shape and 2 mm in height and width.

Then, an electroluminescent layer 502 is formed over the anode 501. Inthis example, the electroluminescent layer 502 has a laminationstructure comprising a hole injecting layer 511; a first light-emittinglayer 512 which has hole injection properties; a second light-emittinglayer 513; an electron transporting layer 514; and an electron injectinglayer 515. The first light-emitting layer 512 is formed by materialswhich can achieve blue emission, specifically, materials which has anemission spectrum with maximum intensity in the wavelength region offrom 400 to 500 nm. In addition, the second light-emitting layer 513 isformed by host materials or guest materials which generatephosphorescent light emission.

First, a substrate provided with the anode 501 is secured with asubstrate holder of a vacuum deposition system in such a way that thesurface provided with the anode 501 is down. Then, Cu-Pc is put into anevaporation source installed in the internal of the vacuum depositionsystem. And then, the hole injection layer 511 is formed to have athickness of 20 nm by vacuum vapor deposition with a resistive heatingmethod.

Then, the first light-emitting layer 512 is formed by a material whichhas excellent hole transportation properties and light-emissionproperties. In this example, α-NPD is deposited in accordance with thesame procedures as those conducted for forming the hole injection layer511 to have a thickness of 30 nm.

And then, the second light-emitting layer 513 is formed. In thisexample, the second light-emitting layer 513 is formed by CBP as hostmaterials and Pt(ppy)acac represented by the structural formula 1 asguest materials which are controlled to be 15 wt % in concentration tohave a thickness of 20 nm by co-evaporation.

Further, the electron transporting layer 514 is formed over the secondlight-emitting layer 513. The electron transporting layer 514 is formedby BCP (bathocuproin) to have a thickness of 20 nm by vapor deposition.CaF₂ is deposited to have a thickness of 2 nm as the electron injectionlayer 515 thereon to complete the electroluminescent layer 502 having alamination structure.

Lastly, a cathode 503 is formed. In this example, the cathode 503 isformed by aluminum (Al) by vapor deposition with a resistive heatingmethod to have a thickness of 100 nm.

Therefore, an electroluminescent element according to the invention isformed. In addition, in the device configuration described in Example 1,each the first light-emitting layer 512 and the second light-emittinglayer 513 can exhibit light emission, so that a device that exhibitswhite light emission as a whole can be formed.

In this example, an anode is formed over a substrate; however, theinvention is not limited thereto. A cathode can be formed over asubstrate. In this case, that is, in case of exchanging an anode tocathode, lamination sequence of the electroluminescent layer describedin this example is reversed.

In this example, the anode 501 is a transparent electrode in order toextract light generated in the electroluminescent layer 502 from theanode 501; however, the invention is not limited thereto. If the cathode503 is formed by a selected material that is suitable for securingtransmittance, light can be extracted from the cathode.

Example 2

In this example, device characteristics of the electroluminescentelement described in Example 1 having the configuration: ITO/Cu-Pc (20nm)/α-NPD (30 nm)/CBP+Pt(ppy)acac: 15 wt % (20 nm)/BCP (30 nm)/CaF (2nm)/Al (100 nm) will be explained. Emission spectrum of theelectroluminescent element having the above described configuration isshown by spectrum 1 in FIG. 8, and FIG. 9. Each plot 1 in FIGS. 10 to 13shows for electric characteristics.

Spectrum 1 in FIG. 8 shows the emission spectrum of theelectroluminescent element having the above described configuration atan applied current of 1 mA (at a luminance of approximately 960 cd/m²).From the result shown by spectrum 1, white light emission can beobtained having three components: blue emission from α-NPD composing thefirst light-emitting layer (around 450 nm); green emission fromphosphorescent light emission of Pt(ppy)acac contained in a secondlight-emitting layer (around 490 nm, around 530 nm); and orange emissionfrom excimer emission of Pt(ppy)acac contained in the secondlight-emitting layer. CIE chromaticity coordinate was (x, y)=(0.346,0.397). The light emission was almost white in appearance.

Ionization potential of the α-NPD used for the first light-emittinglayer and the CBP used for the second light-emitting layer was measured.The α-NPD had ionization potential of approximately 5.3 eV, and the CBPhad that of approximately 5.9 eV. The difference in the ionizationpotential between the α-NPD and the CBP was approximately 0.6 eV.Therefore, preferable condition of the invention, that is, ionizationpotential of at least 0.4 eV, was satisfied. Consequently, it can beconsidered that the fact brought about good white light emission. Inaddition, the measurement of ionization potential was carried out withphotoelectron spectrometer (AC-2) (RIKEN KEIKI Co., Ltd.).

FIG. 9 shows measurement results of each spectrum at different amount ofcurrent flow in the electroluminescent element having the abovedescribed configuration. FIG. 9 shows measurement results at differentamount of current flow denoted by spectrum a (0.1 mA), spectrum b (1mA), and spectrum c (5 mA). Clearly from the measurement results, aspectral shape was hardly changed even when the amount of current flowwas increased (luminance was increased). It can be considered that theelectroluminescent element according to the invention exhibits stablewhite light emission, which is hardly affected by the change of theamount of current flow.

As electric characteristics of the electroluminescent element having theabove described configuration, the luminance-current plot 1 in FIG. 10shows that a luminance of approximately 460 cd/m² was obtained at acurrent density of 10 mA/cm².

The luminance-voltage plot 1 in FIG. 11 shows that a luminance ofapproximately 120 cd/m² was obtained at an applied voltage of 9 V.

The current efficiency-luminance plot 1 in FIG. 12 shows that currentefficiency of approximately 4.6 cd/A was obtained at a luminance of 100cd/m².

The current-voltage plot 1 in FIG. 13 shows that a current flow wasapproximately 0.12 mA at an applied voltage of 9 V.

The obtained amount of Pt in the above described electroluminescentelement was 21 ng by quantitative determination by Inductively CoupledPlasma-Mass Spectrometry (ICP-MS). The obtained atomic concentration perunit area was 5.4×10¹⁴ atoms/cm² by converting the amount of Pt.

Further, depth profiling of Pt concentration was conducted by SecondaryIon Mass Spectrometry (SIMS), and the above described amount of Pt wasconverted into the original amount, then, calculated the concentrationof Pt per unit volume. In consequence, the maximum value of Ptconcentration per unit was approximately 2.0×10²⁰ atoms/cm³, and3.3×10⁻⁴ mol/cm³ at mol concentration. Therefore, it can be consideredthat excimer emission becomes possible if the concentration ofphosphorescent materials forming excimer is in the range of from 10⁻⁴ to10⁻³ mol/cm³.

As mentioned above, because of the fact that the maximum value of Ptconcentration per unit volume is approximately 2.0×10²⁰ atoms/cm³, theaverage volume of one Pt complex is 5.0×10⁻²⁷ m³/atom. In case that thePt complex is dispersed evenly, the Pt complex is dispersed inphosphorescent materials in the proportion of one Pt complex to 1.7cubic nm by volume. Therefore, the distance between metal atoms eachother of phosphorescent materials (in this example, Pt atom) isapproximately 17 Å. Hence, the distance between central metals eachother of phosphorescent materials is preferably at most 20 Å accordingto the invention.

Comparative Example 1

Correspondingly, each spectrum 2 and spectrum 3 in FIG. 8 shows emissionspectrum measured from an electroluminescent element in which alight-emitting layer comprise Pt(ppy)acac at different concentrationfrom that described in Example 1. The spectrum 2 shows a measurementresult in the case that concentration of Pt(ppy)acac is 7.9 wt %. Thespectrum 3 shows a measurement result in the case that concentration ofPt(ppy)acac is 2.5 wt %. In each case, the spectrum was obtained at thecurrent flow of 1 mA.

As shown by spectrum 3, in case that Pt(ppy)acac is contained atconcentration of 2.5 wt %, blue emission from α-NPD composing a firstlight-emitting layer (around 450 nm) and green emission from Pt(ppy)acaccontained in a second light-emitting layer (around 490 nm, around 530nm) were only observed, and white light emission has not resulted. Asshown in spectrum 2, in case that Pt(ppy)acac is contained atconcentration of 7.9 wt %, a slight of excimer emission was in thespectrum as a shoulder at the vicinity of 560 nm; however the peak wasinsufficient, consequently, excellent white light emission could not beobserved.

Further, current characteristics were measured from the devices. Eachplot 2 in FIGS. 10 to 13 shows measurement results from the devicecontaining Pt(ppy)acac at concentration of 7.9 wt %. Each plot 3 inFIGS. 10 to 13 shows measurement results from the device containingPt(ppy)acac at concentration of 2.5 wt %.

The luminance-voltage characteristics in FIG. 10 show that a luminanceof approximately 180 cd/m² was obtained from the device containingPt(ppy)acac at concentration of 7.9 wt % and a luminance ofapproximately 115 cd/m² was obtained from the device containingPt(ppy)acac at concentration of 2.5 wt % at a current density of 10mA/cm², respectively.

The luminance-voltage characteristics in FIG. 11 show that a luminanceof approximately 93 cd/m² was obtained form the device containingPt(ppy)acac at concentration of 7.9 wt % and a luminance ofapproximately 73 cd/m² was obtained from the device containingPt(ppy)acac at concentration of 2.5 wt % at an applied voltage of 9 V,respectively.

The current efficiency-luminance characteristics in FIG. 12 show that acurrent efficiency of approximately 1.8 cd/A was obtained from thedevice containing Pt(ppy)acac at concentration of 7.9 wt % and a currentefficiency of approximately 1.1 cd/A was obtained from the devicecontaining Pt(ppy)acac at concentration of 2.5 wt % at the luminance of100 cd/m², respectively.

The current-voltage characteristics in FIG. 13 show that a current flowwas approximately 0.21 mA in the device containing Pt(ppy)acac atconcentration of 7.9 wt % and a current flow was approximately 0.27 mAin the device containing Pt(ppy)acac at concentration of 2.5 wt % at anapplied voltage of 9 V, respectively.

The above measurement results (especially, from the result of thecurrent-voltage characteristics shown in FIG. 13) provide the fact thatthe electroluminescent element according to the invention containingPt(ppy)acac as guest materials in high concentration (15 wt %) has thesame level of electric characteristics as those of theelectroluminescent element containing Pt(ppy)acac as guest materials insuch low concentration (7.9 wt %, 2.5 wt %).

Example 3

In this example, an example for manufacturing a light-emitting device(top emission structure) having an electroluminescent element accordingto the present invention which exhibits white light emission over asubstrate having an insulating surface will be explained with referenceto FIG. 6. As used herein, the term “top emission structure” refers to astructure that light is extracted from the opposite side of a substratehaving an insulating surface.

FIG. 6A is a top view of a light-emitting device. FIG. 6B is across-sectional view of FIG. 6A taken along the line A-A′. Referencenumeral 601 indicated by a dotted line denotes a source signal linedriver circuit; 602, a pixel portion; 603, a gate signal line drivercircuit; 604, a transparent sealing substrate; 605, a first sealingagent; and 607, a second sealing agent. The inside surrounded by thefirst sealing agent 605 is filled with the transparent second sealingagent 607. In addition, the first sealing agent 605 contains a gap agentfor spacing between substrates.

Reference 608 denotes a connecting wiring for transmitting signalsinputted to the source signal line driver circuit 601 and the gatesignal line driver circuit 603. The wiring receives video signals orclock signals from an FPC (flexible printed circuit) 609 serving as anexternal input terminal. Although only FPC is illustrated in thedrawing, a PWB (printed wirings board) may be attached to the FPC.

Then, a cross-sectional structure will be explained with reference toFIG. 6B. A driver circuit and a pixel portion are formed over asubstrate 610. In FIG. 6B, the source signal line driver circuit 601 andthe pixel portion 602 are illustrated as a driver circuit.

The source signal line driver circuit 601 is provided with a CMOScircuit formed by combining an n-channel TFT 623 and a p-channel TFT624. A TFT for forming a driver circuit may be formed by a known CMOS,PMOS, or NMOS circuit. In this example, a driver integrated type, thatis, a driver circuit is formed over a substrate, is described, but notexclusively, the driver circuit can be formed outside instead of over asubstrate. In addition, the structure of a TFT using a polysilicon filmas an active layer is not especially limited. A top gate TFT or a bottomgate TFT can be adopted.

The pixel portion 602 is composed of a plurality of pixels including aswitching TFT 611, a current control TFT 612, and a first electrode(anode) 613 connected to the drain of the current control TFT 612. Thecurrent control TFT 612 may be either an n-channel TFT or a p-channelTFT. In case that the current control TFT 612 is connected to an anode,the TFT is preferably a p-channel TFT. In FIG. 6B, a cross-sectionalstructure of only one of thousands of pixels is illustrated to show anexample that two TFTs are used for the pixel. However, three or morenumbers of TFTs can be appropriately used.

Since the first electrode (anode) 613 is directly in contact with thedrain of a TFT, a bottom layer of the first electrode (anode) 613 ispreferably formed by a material capable of making an ohmic contact withthe drain formed by silicon, and a top layer which is in contact with alayer containing an organic compound is preferably formed by a materialhaving large work functions. In case of forming the first electrode(anode) by three layers structure comprising a titanium nitride film, afilm containing aluminum as its main component, and a titanium nitridefilm, the first electrode (anode) can reduce resistance as a wiring,make a favorable ohmic contact, and function as an anode. Further, thefirst electrode (anode) 613 can be formed by a single layer such as atitanium nitride film, a chromium film, a tungsten film, a zinc film, ora platinum film; or a lamination layer composed of three or more layers.

Insulator (also referred to as a bank) 614 is formed at the edge of thefirst electrode (anode) 613. The insulator 614 may be formed by anorganic resin film or an insulating film containing silicon. In thisexample, an insulator is formed by a positive type photosensitiveacrylic film as the insulator 614 in the shape as illustrated in FIG. 6.

In order to make favorable coverage, an upper edge portion or a loweredge portion of the insulator 614 is formed to have a curved face havinga radius of curvature. For example, in case that positive typephotosensitive acrylic is used as a material for the insulator 614, onlyupper edge portion of the insulator 614 is preferably having a radius ofcurvature (from 0.2 to 3 μm). As the insulator 614, either a negativetype photosensitive resin that becomes insoluble to etchant by light ora positive type photosensitive resin that becomes dissoluble to etchantby light can be used.

Further, the insulator 614 may be covered by a protective film formed byan aluminum nitride film, an aluminum nitride oxide film, a thin filmcontaining carbon as its main component, or a silicon nitride film.

An electroluminescent layer 615 is selectively formed over the firstelectrode (anode) 613 by vapor deposition. Moreover, a second electrode(cathode) 616 is formed over the electroluminescent layer 615. As thecathode, a material having a small work function (Al, Ag, Li, Ca; oralloys of these elements such as Mg:Ag, Mg:In, or Al:Li; or CaN) can beused.

In order to pass light, the second electrode (cathode) 616 is formed bya lamination layer of a thin metal film having small work functions anda transparent conductive film (ITO, IZO, ZnO, or the like). Anelectroluminescent element 618 is thus formed comprising the firstelectrode (anode) 613, the electroluminescent layer 615, and the secondelectrode (cathode) 616.

In this example, the electroluminescent layer 615 is formed by alamination structure explained in Example 1. That is, theelectroluminescent layer 615 is formed by stacking sequentially Cu-Pc asa hole injecting layer (20 nm), α-NPD as a first light-emitting layerhaving hole transporting properties (30 nm), CBP+Pt(ppy)acac:15 wt % (20nm) as a second light-emitting layer, and BCP as an electrontransporting layer (30 nm). In addition, an electron injecting layer(CaF₂) is unnecessary in the device since the second electrode (cathode)is formed by a thin film metal film having small work functions.

Thus formed electroluminescent element 618 exhibits white lightemission. In addition, a color filter comprising a coloring layer 631and a light shielding layer (BM) 632 is provided to realize full color(for simplification, an over coat layer is not illustrated).

In order to seal the electroluminescent element 618, a transparentprotective lamination layer 617 is formed. The transparent protectivelamination layer 617 comprises a first inorganic insulating film, astress relaxation film, and a second inorganic insulating film. As thefirst inorganic insulating film and the second inorganic insulatingfilm, a silicon nitride film, a silicon oxide film, a silicon oxynitridefilm (composition ratio: N<O), a silicon nitride oxide film (compositionratio: N>O), or a thin film containing carbon as its main component (forexample, a DLC film or a CN film) can be used. These inorganicinsulating films have high blocking properties against moisture.However, when the film thickness is increased, film stress is alsoincreased, consequently, film peeling is easily occurred.

By interposing a stress relaxation film between the first inorganicinsulating film and the second inorganic insulating film, moisture canbe absorbed and stress can be relaxed. Even when fine holes (such as pinholes) are existed on the first inorganic insulating film during formingthe film for any reason, the stress relaxation film can fill the fineholes. The second inorganic insulating film formed over the stressrelaxation film gives the transparent protective lamination filmexcellent blocking properties against moisture or oxygen.

Materials having smaller stress than that of an inorganic insulatingfilm and hygroscopic properties are preferably used for the stressrelaxation film. In addition, a material that is transparent to light ispreferable. As the stress relaxation film, a film containing an organiccompound such as α-NPD, BCP, MTDATA, or Alq₃ can be used. These filmshave hygroscopic properties and are almost transparent in case of havingthin film thickness. Further, MgO, SrO₂, or SrO can be used as thestress relaxation film since they have hygroscopic properties andtranslucency, and can be formed into a thin film by vapor deposition.

In this example, a silicon nitride film which is formed by vapordeposition using a silicon target in the atmosphere containing nitrogenand argon to have high blocking properties against impurities such asmoisture or alkaline metals is used as the first inorganic insulatingfilm or the second inorganic insulating film. Alq₃ is deposited to forma thin film as the stress relaxation film by vapor deposition. In orderto pass light through the transparent protective lamination layer, thetotal film thickness of the transparent protective lamination layer ispreferably formed to be thin as possible.

In order to seal the electroluminescent element 618, the sealingsubstrate 604 is pasted with the first sealing agent 605 and the secondsealing agent 607 in an inert gas atmosphere. Epoxy resin is preferablyused for the first sealing agent 605 and the second sealing agent 607.It is desirable that the first sealing agent 605 and the second sealingagent 607 inhibit moisture or oxygen as possible.

In this example, as a material for the sealing substrate 604, a plasticsubstrate formed by FRP (Fiberglass-Reinforced Plastics), PVF (polyvinylfluoride), Myler, polyester, acrylic, or the like can be used besides aglass substrate or a quartz substrate. After pasting the sealingsubstrate 604 with the first sealing agent 605 and the second sealingagent 607, a third sealing agent can be provided to seal the side face(exposed face).

By encapsulating the electroluminescent element 618 in the first sealingagent 605 and the second sealing agent 607, the electroluminescentelement 618 can be shielded completely from outside to prevent moistureor oxygen that brings deterioration of the electroluminescent layer 615from penetrating into the electroluminescent element 618. Therefore, ahigh reliable light-emitting device can be obtained.

If a transparent conductive film is used as the first electrode (anode)613, a dual emission device can be manufactured.

The light-emitting device according to this example can be practiced byutilizing not only the device configuration of the electroluminescentdevice explained in Example 1 but also combining the configuration ofthe electroluminescent device formed according to the invention withthat explained in Example 1.

Example 4

Various electric appliances completed by using a light-emitting devicehaving an electroluminescent element according to the present inventionwill be explained in this example.

Given as examples of such electric appliances manufactured by using thelight-emitting device having the electroluminescent element according tothe invention: a video camera, a digital camera, a goggles-type display(head mount display), a navigation system, a sound reproduction device(a car audio equipment, an audio set and the like), a laptop personalcomputer, a game machine, a portable information terminal (a mobilecomputer, a cellular phone, a portable game machine, an electronic book,or the like), an image reproduction device including a recording medium(more specifically, a device which can reproduce a recording medium suchas a digital versatile disc (DVD) and so forth, and includes a displayfor displaying the reproduced image), or the like. FIGS. 7A to 7G showvarious specific examples of such electric appliances.

FIG. 7A illustrates a display device which includes a frame 7101, asupport table 7102, a display portion 7103, a speaker portion 7104, avideo input terminal 7105, or the like. The light-emitting device usingthe electroluminescent element according to the invention can be usedfor the display portion 7103. The display device is including all of thedisplay devices for displaying information, such as a personal computer,a receiver of TV broadcasting, and an advertising display.

FIG. 7B illustrates a laptop computer which includes a main body 7201, acasing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing mouse 7206, or the like. Thelight-emitting device using the electroluminescent element according tothe invention can be used to the display portion 7203.

FIG. 7C illustrates a mobile computer which includes a main body 7301, adisplay portion 7302, a switch 7303, an operation key 7304, an infraredport 7305, or the like. The light-emitting device using theelectroluminescent element according to the invention can be used to thedisplay portion 7302.

FIG. 7D illustrates an image reproduction device including a recordingmedium (more specifically, a DVD reproduction device), which includes amain body 7401, a casing 7402, a display portion A 7403, another displayportion B 7404, a recording medium (DVD or the like) reading portion7405, an operation key 7406, a speaker portion 7407 or the like. Thedisplay portion A 7403 is used mainly for displaying image information,while the display portion B 7404 is used mainly for displaying characterinformation. The light-emitting device using the electroluminescentelement according to the invention can be used to the display portion A7403 and the display portion B 7404. Note that the image reproductiondevice including a recording medium further includes a domestic gamemachine or the like.

FIG. 7E illustrates a goggle type display (head mounted display), whichincludes a main body 7501, a display portion 7502, and an arm portion7503. The light-emitting device using the electroluminescent elementaccording to the invention can be used to the display portion 7502.

FIG. 7F illustrates a video camera which includes a main body 7601, adisplay portion 7602, an casing 7603, an external connecting port 7604,a remote control receiving portion 7605, an image receiving portion7606, a battery 7607, a sound input portion 7608, an operation key 7609,an eyepiece portion 7610 or the like. The light-emitting device usingthe electroluminescent element according to the invention can be used tothe display portion 7602.

FIG. 7G illustrates a cellular phone which includes a main body 7701, acasing 7702, a display portion 7703, a sound input portion 7704, a soundoutput portion 7705, an operation key 7706, an external connecting port7707, an antenna 7708, or the like. The light-emitting device using theelectroluminescent element according to the invention can be used to thedisplay portion 7703. Note that the display portion 7703 can reducepower consumption of the cellular phone by displaying white-coloredcharacters on a black-colored background.

Additionally, the electroluminescent element according to the inventioncan be applied to lighting equipment, wall of establishment, or the likewhich serves as a surface light source.

As mentioned above, an application range of the light-emitting deviceusing the electroluminescent element according to the invention isextremely wide. Further, the electroluminescent element according to theinvention can be easily controlled the color balance in white lightemission (white balance). Therefore, a well color balanced display canbe realized in electric appliances in various fields by applying theelectroluminescent element according to the invention thereto.

Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention hereinafterdescribed, they should be construed as being included therein.

What is claimed is:
 1. A light-emitting device comprising: a firstelectrode; a second electrode; and a light-emitting layer between thefirst electrode and the second electrode; wherein the light-emittinglayer comprises a phosphorescent material at a concentration of at least12.5 wt %.
 2. The light-emitting device according to claim 1, furthercomprising: a second light-emitting layer between the first electrodeand the light-emitting layer.
 3. The light-emitting device according toclaim 2, wherein the light-emitting layer and the second light-emittinglayer are in contact with each other.
 4. The light-emitting deviceaccording to claim 2, wherein a wavelength in an emission peak of thesecond light-emitting layer is smaller than a wavelength in an emissionpeak of the light-emitting layer.
 5. The light-emitting device accordingto claim 1, wherein one of the first electrode and the second electrodehas light-shielding property.
 6. The light-emitting device according toclaim 1, wherein the light-emitting device can emit white light.
 7. Thelight-emitting device according to claim 1, wherein the phosphorescentmaterial is an organometal complex with platinum.
 8. The light-emittingdevice according to claim 1, wherein the light-emitting layer comprisesthe phosphorescent material at a concentration of 40 wt % or less. 9.The light-emitting device according to claim 1, further comprising: acolor filter overlapping with the light-emitting layer.
 10. A lightingequipment comprising the light-emitting device according to claim
 1. 11.A display device comprising a display portion that the light-emittingdevice according to claim
 1. 12. A light-emitting device comprising: afirst electrode; a second electrode; and a light-emitting layer betweenthe first electrode and the second electrode; wherein the light-emittinglayer comprises one of a hole transporting material and a electrontransporting material, and wherein the light-emitting layer comprises aphosphorescent material at a concentration of at least 12.5 wt %. 13.The light-emitting device according to claim 12, further comprising: asecond light-emitting layer between the first electrode and thelight-emitting layer.
 14. The light-emitting device according to claim13, wherein the light-emitting layer and the second light-emitting layerare in contact with each other.
 15. The light-emitting device accordingto claim 13, wherein a wavelength in an emission peak of the secondlight-emitting layer is smaller than a wavelength in an emission peak ofthe light-emitting layer.
 16. The light-emitting device according toclaim 12, wherein one of the first electrode and the second electrodehas light-shielding property.
 17. The light-emitting device according toclaim 12, wherein the light-emitting device can emit white light. 18.The light-emitting device according to claim 12, wherein thephosphorescent material is an organometal complex with platinum.
 19. Thelight-emitting device according to claim 12, wherein the light-emittinglayer comprises the phosphorescent material at a concentration of 40 wt% or less.
 20. The light-emitting device according to claim 12, furthercomprising: a color filter overlapping with the light-emitting layer.21. A lighting equipment comprising the light-emitting device accordingto claim
 12. 22. A display device comprising a display portion that thelight-emitting device according to claim
 12. 23. A light-emitting devicecomprising: a first electrode; a second electrode; and a firstlight-emitting layer between the first electrode and the secondelectrode; a second light-emitting layer between the first electrode andthe first light-emitting layer; wherein the first light-emitting layercomprises a phosphorescent material at a concentration of at least 12.5wt %, and wherein the second light-emitting layer comprises afluorescent material.
 24. The light-emitting device according to claim23, wherein the first light-emitting layer and the second light-emittinglayer are in contact with each other.
 25. The light-emitting deviceaccording to claim 23, wherein a wavelength in an emission peak of thesecond light-emitting layer is smaller than a wavelength in an emissionpeak of the first light-emitting layer.
 26. The light-emitting deviceaccording to claim 23, wherein one of the first electrode and the secondelectrode has light-shielding property.
 27. The light-emitting deviceaccording to claim 23, wherein the light-emitting device can emit whitelight.
 28. The light-emitting device according to claim 23, wherein thephosphorescent material is an organometal complex with platinum.
 29. Thelight-emitting device according to claim 23, wherein the firstlight-emitting layer comprises the phosphorescent material at aconcentration of 40 wt % or less.
 30. The light-emitting deviceaccording to claim 23, further comprising: a color filter overlappingwith the first light-emitting layer.
 31. A lighting equipment comprisingthe light-emitting device according to claim
 23. 32. A display devicecomprising a display portion that the light-emitting device according toclaim
 23. 33. The light-emitting device according to claim 23, whereinthe fluorescent material is a blue fluorescent material.
 34. Thelight-emitting device according to claim 23, wherein the first electrodeis an anode, and wherein the second electrode is a cathode.
 35. Thelight-emitting device according to claim 23, wherein the fluorescentmaterial is a blue fluorescent material, wherein the first electrode isan anode, and wherein the second electrode is a cathode.